Activating Injury-Responsive Genes with Hypoxia Enhances Axon Regeneration through Neuronal HIF-1α

Nucleus pulposus cell-mimicking nanoparticles for cell-specific HIF1A editing to modulate SASP-mediated disc inflammation via autophagy activation

Traditional methods of plasmid delivery, including viral vectors, lipofection, and electroporation, are widely used for gene editing but have limitations, such as cellular toxicity, limited transfection efficiency in primary cells, and nonspecific side effects. Here, we report the development of nucleus pulposus cell (NPC)-mimicking nanoparticles (HIF1A@NNP) with an NPC membrane as the shell and pcDNA3.1+-rHIF1A encapsulated in the core via extrusion. HIF1A@NNP exhibited a protein expression pattern similar to that of the NPC membrane and displayed a typical vesicle profile. Compared to liposomes and lentiviruses, HIF1A@NNP overexpressed HIF1A in NPCs while improving cell viability. HIF1A@NNP was more readily internalized by NPCs than by other cell types, with fewer effects on vascularization, nerve growth, and macrophage polarization than HIF1A overexpression using lipo3000. HIF1A@NNP reduced the apoptotic rate and inhibited the senescent phenotype, as evidenced by reduced DNA damage, lower levels of senescence-related proteins, and fewer SA-β-Gal-positive cells. HIF1A@NNP induced a senescence-associated secretory phenotype (SASP), which enhanced macrophage migration and M1 polarization. Additionally, HIF1A@NNP activated autophagy in NPCs. In summary, HIF1A@NNP demonstrated satisfactory biocompatibility, alleviated the SASP, and inhibited SASP-mediated macrophage recruitment and inflammatory polarization, leading to reduced disc degeneration and providing a promising strategy for combating intervertebral disc degeneration. STATEMENT OF SIGNIFICANCE: Conventional plasmid delivery methods like viral vectors, lipofection, and electroporation struggle with cellular toxicity and inefficiency in primary cells. Non-cell-specific HIF1A activation via these methods may exacerbate inflammation and pain, as HIF1A drives angiogenesis and dendritic ingrowth into the disc. Thus, a cell-specific delivery strategy could circumvent such adverse effects. Our study introduces HIF1A@NNP, a nanoparticle mimicking nucleus pulposus cells (NPCs), with an NPC membrane shell encapsulating pcDNA3.1+-rHIF1A. It preferentially targets NPCs, achieving superior HIF1A overexpression and cell viability compared to liposomes and lentiviruses. This represents a highly promising and potentially transformative approach against intervertebral disc degeneration.

Inhibition of HIF-prolyl hydroxylase promotes renal tubule regeneration via the reprogramming of renal proximal tubular cells

The ability of the mammalian kidney to repair or regenerate after acute kidney injury (AKI) is very limited. The maladaptive repair of AKI promotes progression to chronic kidney disease (CKD). Therefore, new strategies to promote the repair/regeneration of injured renal tubules after AKI are urgently needed. Hypoxia has been shown to induce heart regeneration in adult mice. However, it is unknown whether hypoxia can induce kidney regeneration after AKI. In this study, we used a prolyl hydroxylase domain inhibitor (PHDI), MK-8617, to mimic hypoxic conditions and found that MK-8617 significantly ameliorated ischemia reperfusion injury (IRI)-induced AKI. We also showed that MK-8617 dramatically facilitated renal tubule regeneration by promoting the proliferation of renal proximal tubular cells (RPTCs) after IRI-induced AKI. We then performed bulk mRNA sequencing and discovered that multiple nephrogenesis-related genes were significantly upregulated with MK-8617 pretreatment. We also showed that MK-8617 may alleviate proximal tubule injury by stabilizing the HIF-1α protein specifically in renal proximal tubular cells. Furthermore, we demonstrated that MK-8617 promotes the reprogramming of renal proximal tubular cells to Sox9+ renal progenitor cells and the regeneration of renal proximal tubules. In summary, we report that the inhibition of prolyl hydroxylase improves renal proximal tubule regeneration after IRI-induced AKI by promoting the reprogramming of renal proximal tubular cells to Sox9+ renal progenitor cells.

The mechanotransduction protein STOML3 is required for proprioceptor plasticity following peripheral nerve regeneration

Nerve regeneration is associated with the plasticity of sensory neurons such that even muscle afferents directed to the skin form mechanosensitive receptive fields appropriate for the new target. STOML3 is an essential mechanotransduction component in many cutaneous mechanoreceptors. Here, we asked whether STOML3 is required for functional and anatomical plasticity following peripheral nerve regeneration. We used a cross-anastomosis model adapted to the mouse, in which the medial gastrocnemius nerve was redirected to innervate hairy skin previously occupied by the sural nerve. We recorded from muscle afferents innervating the skin and found that in wild-type mice their receptive properties were largely identical to normal skin mechanoreceptors. However, in mice lacking STOML3, muscle afferents largely failed to form functional mechanosensitive receptive fields, despite making anatomically appropriate endings in the skin. Our tracing experiments demonstrated that muscle afferents from both wild-type and stoml3 mutant mice display remarkable anatomical plasticity, forming new somatotopically appropriate synaptic terminals in the region of the dorsal horn representing the sural nerve territory. The dramatic reduction in stimulus-evoked activity from the cross-anastomosed gastrocnemius nerve in stoml3 mutant mice did not prevent central anatomical plasticity. Our results have identified a molecular factor required for functional plasticity following peripheral nerve injury.

Unleashing metabolic power for axonal regeneration

Axon regeneration requires the mobilization of intracellular resources, including proteins, lipids, and nucleotides. After injury, neurons need to adapt their metabolism to meet the biosynthetic demands needed to achieve axonal regeneration. However, the exact contribution of cellular metabolism to this process remains elusive. Insights into the metabolic characteristics of proliferative cells may illuminate similar mechanisms operating in axon regeneration; therefore, unraveling previously unappreciated roles of metabolic adaptation is critical to achieving neuron regrowth, which is connected to the therapeutic strategies for neurological conditions necessitating nerve repairs, such as spinal cord injury and stroke. Here, we outline the metabolic role in axon regeneration and discuss factors enhancing nerve regrowth, highlighting potential novel metabolic treatments for restoring nerve function.

Hypoxia-induced conversion of sensory Schwann cells into repair cells is regulated by HDAC8

After a peripheral nerve injury, Schwann cells (SCs), the myelinating glia of the peripheral nervous system, convert into repair cells that foster axonal regrowth, and then remyelinate or re-ensheath regenerated axons, thereby ensuring functional recovery. The efficiency of this mechanism depends however on the time needed for axons to regrow. Here, we show that ablation of histone deacetylase 8 (HDAC8) in SCs accelerates the regrowth of sensory axons and sensory function recovery. We found that HDAC8 is specifically expressed in sensory SCs and regulates the E3 ubiquitin ligase TRAF7, which destabilizes hypoxia-inducible factor 1-alpha (HIF1α) and counteracts the phosphorylation and upregulation of c-Jun, a major inducer of the repair SC phenotype. Our study indicates that this phenotype switch is regulated by different mechanisms in sensory and motor SCs and is accelerated by HDAC8 downregulation, which promotes sensory axon regeneration and sensory function recovery.

Local glycolysis supports injury-induced axonal regeneration

Successful axonal regeneration following injury requires the effective allocation of energy. How axons withstand the initial disruption in mitochondrial energy production caused by the injury and subsequently initiate regrowth is poorly understood. Transcriptomic data showed increased expression of glycolytic genes after optic nerve crush in retinal ganglion cells with the co-deletion of Pten and Socs3. Using retinal cultures in a multicompartment microfluidic device, we observed increased regrowth and enhanced mitochondrial trafficking in the axons of Pten and Socs3 co-deleted neurons. While wild-type axons relied on mitochondrial metabolism, after injury, in the absence of Pten and Socs3, energy production was supported by local glycolysis. Specific inhibition of lactate production hindered injury survival and the initiation of regrowth while slowing down glycolysis upstream impaired regrowth initiation, axonal elongation, and energy production. Together, these observations reveal that glycolytic ATP, combined with sustained mitochondrial transport, is essential for injury-induced axonal regrowth, providing new insights into the metabolic underpinnings of axonal regeneration.

Genes in Axonal Regeneration

This review explores the molecular and genetic underpinnings of axonal regeneration and functional recovery post-nerve injury, emphasizing its significance in reversing neurological deficits. It presents a systematic exploration of the roles of various genes in axonal regrowth across peripheral and central nerve injuries. Initially, it highlights genes and gene families critical for axonal growth and guidance, delving into their roles in regeneration. It then examines the regenerative microenvironment, focusing on the role of glial cells in neural repair through dedifferentiation, proliferation, and migration. The concept of "traumatic microenvironments" within the central nervous system (CNS) and peripheral nervous system (PNS) is discussed, noting their impact on regenerative capacities and their importance in therapeutic strategy development. Additionally, the review delves into axonal transport mechanisms essential for accurate growth and reinnervation, integrating insights from proteomics, genome-wide screenings, and gene editing advancements. Conclusively, it synthesizes these insights to offer a comprehensive understanding of axonal regeneration's molecular orchestration, aiming to inform effective nerve injury therapies and contribute to regenerative neuroscience.

Aryl hydrocarbon receptor restricts axon regeneration of DRG neurons in response to injury

Injured neurons sense environmental cues to balance neural protection and axon regeneration, but the mechanisms are unclear. Here, we unveil aryl hydrocarbon receptor (AhR), a ligand-activated bHLH-PAS transcription factor, as a molecular sensor and key regulator of acute stress response at the expense of axon regeneration. We demonstrate responsiveness of DRG sensory neurons to AhR signaling, which functions to inhibit axon regeneration. Conditional Ahr deletion in neurons accelerates axon regeneration after sciatic nerve injury. Ahr deletion partially mimics the conditioning lesion in priming DRG to initiate axonogenesis gene programs; upon peripheral axotomy, Ahr ablation suppresses inflammation and stress signaling while augmenting pro-growth pathways. Moreover, comparative transcriptomics revealed signaling interactions between AhR and HIF-1α, two structurally related bHLH-PAS α units that share the dimerization partner Arnt/HIF-1β. Functional assays showed that the growth advantage of AhR-deficient DRG neurons requires HIF-1α; but in the absence of Arnt, DRG neurons can still mount a regenerative response. We further unveil a link between bHLH-PAS transcription factors and DNA hydroxymethylation in response to peripheral axotomy, while RNA-seq of DRG neurons and neuronal single cell RNA-seq analysis revealed a link of AhR regulon to RNA regulation and integrated stress response (ISR). Altogether, AhR activation favors stress coping and inflammation at the expense of axon regeneration; targeting AhR has the potential to enhance nerve repair.

Global ischemia induces stemness and dedifferentiation in human adult cardiomyocytes after cardiac arrest

Global ischemia has been shown to induce cardiac regenerative response in animal models. One of the suggested mechanisms behind cardiac regeneration is dedifferentiation of cardiomyocytes. How human adult cardiomyocytes respond to global ischemia is not fully known. In this study, biopsies from the left ventricle (LV) and the atrioventricular junction (AVj), a potential stem cell niche, were collected from multi-organ donors with cardiac arrest (N = 15) or without cardiac arrest (N = 6). Using immunohistochemistry, we investigated the expression of biomarkers associated with stem cells during cardiomyogenesis; MDR1, SSEA4, NKX2.5, and WT1, proliferation markers PCNA and Ki67, and hypoxia responsive factor HIF1α. The myocyte nuclei marker PCM1 and cardiac Troponin T were also included. We found expression of cardiac stem cell markers in a subpopulation of LV cardiomyocytes in the cardiac arrest group. The same cells showed a low expression of Troponin T indicating remodeling of cardiomyocytes. No such expression was found in cardiomyocytes from the control group. Stem cell biomarker expression in AVj was more pronounced in the cardiac arrest group. Furthermore, co-expression of PCNA and Ki67 with PCM1 was only found in the cardiac arrest group in the AVj. Our results indicate that a subpopulation of human cardiomyocytes in the LV undergo partial dedifferentiation upon global ischemia and may be involved in the cardiac regenerative response together with immature cardiomyocytes in the AVj.

Enhancing intraneural revascularization following peripheral nerve injury through hypoxic Schwann-cell-derived exosomes: an insight into endothelial glycolysis

BACKGROUND

Endothelial cell (EC)-driven intraneural revascularization (INRV) and Schwann cells-derived exosomes (SCs-Exos) both play crucial roles in peripheral nerve injury (PNI). However, the interplay between them remains unclear. We aimed to elucidate the effects and underlying mechanisms of SCs-Exos on INRV following PNI.

RESULTS

We found that GW4869 inhibited INRV, as well as that normoxic SCs-Exos (N-SCs-Exos) exhibited significant pro-INRV effects in vivo and in vitro that were potentiated by hypoxic SCs-Exos (H-SCs-Exos). Upregulation of glycolysis emerged as a pivotal factor for INRV after PNI, as evidenced by the observation that 3PO administration, a glycolytic inhibitor, inhibited the INRV process in vivo and in vitro. H-SCs-Exos more significantly enhanced extracellular acidification rate/oxygen consumption rate ratio, lactate production, and glycolytic gene expression while simultaneously suppressing acetyl-CoA production and pyruvate dehydrogenase E1 subunit alpha (PDH-E1α) expression than N-SCs-Exos both in vivo and in vitro. Furthermore, we determined that H-SCs-Exos were more enriched with miR-21-5p than N-SCs-Exos. Knockdown of miR-21-5p significantly attenuated the pro-glycolysis and pro-INRV effects of H-SCs-Exos. Mechanistically, miR-21-5p orchestrated EC metabolism in favor of glycolysis by targeting von Hippel-Lindau/hypoxia-inducible factor-1α and PDH-E1α, thereby enhancing hypoxia-inducible factor-1α-mediated glycolysis and inhibiting PDH-E1α-mediated oxidative phosphorylation.

CONCLUSION

This study unveiled a novel intrinsic mechanism of pro-INRV after PNI, providing a promising therapeutic target for post-injury peripheral nerve regeneration and repair.

Hypoxic preconditioning accelerates the healing of ischemic intestinal injury by activating HIF-1α/PPARα pathway-mediated fatty acid oxidation

Hypoxic preconditioning (HPC) has been shown to improve organ tolerance to subsequent severe hypoxia or ischemia. However, its impact on intestinal ischemic injury has not been well studied. In this study, we evaluated the effects of HPC on intestinal ischemia in rats. Intestinal rehabilitation, levels of fatty acid oxidation (FAO) by-products, intestinal stem cells (ISCs), levels of hypoxia-inducible factor 1 subunit α (HIF-1α) and its downstream genes such as peroxisome proliferator-activated receptor α (PPARα), and carnitine palmitoyltransferase 1a (CPT1A) were assessed at distinct time intervals following intestinal ischemia with or without the interference of HIF-1α. Our data showed that HPC facilitates the restoration of the intestinal structure and enhances the FAO, by boosting intestinal stem cells. Additionally, HIF-1α, PPARα, and CPT1A mRNA and their protein levels were generally up-regulated in the small intestine of HPC rats as compared to the control group. Our vitro experiment also shows low-oxygen induces highly levels of HIF-1α and its downstream genes, with a concurrent increase in FAO products in IEC-6 cells. Furthermore, the above phenomenon could be reversed by silencing HIF-1α. In conclusion, we hypothesize that HPC can stimulate the activation of intestinal stem cells via HIF-1α/PPARα pathway-mediated FAO, thereby accelerating the healing process post ischemic intestinal injury.

Skin derived precursors induced Schwann cells mediated tissue engineering-aided neuroregeneration across sciatic nerve defect

A central question in neural tissue engineering is how the tissue-engineered nerve (TEN) translates detailed transcriptional signals associated with peripheral nerve regeneration into meaningful biological processes. Here, we report a skin-derived precursor-induced Schwann cell (SKP-SC)-mediated chitosan/silk fibroin-fabricated tissue-engineered nerve graft (SKP-SCs-TEN) that can promote sciatic nerve regeneration and functional restoration nearly to the levels achieved by autologous nerve grafts according to behavioral, histological, and electrophysiological evidence. For achieving better effect of neuroregeneration, this is the first time to jointly apply a dynamic perfusion bioreactor and the ascorbic acid to stimulate the SKP-SCs secretion of extracellular matrix (ECM). To overcome the limitation of traditional tissue-engineered nerve grafts, jointly utilizing SKP-SCs and their ECM components were motivated by the thought of prolongating the effect of support cells and their bioactive cues that promote peripheral nerve regeneration. To further explore the regulatory model of gene expression and the related molecular mechanisms involved in tissue engineering-aided peripheral nerve regeneration, we performed a cDNA microarray analysis of gene expression profiling, a comprehensive bioinformatics analysis and a validation study on the grafted segments and dorsal root ganglia tissues. A wealth of transcriptomic and bioinformatics data has revealed complex molecular networks and orchestrated functional regulation that may be responsible for the effects of SKP-SCs-TEN on promoting peripheral nerve regeneration. Our work provides new insights into transcriptomic features and patterns of molecular regulation in nerve functional recovery aided by SKP-SCs-TEN that sheds light on the broader possibilities for novel repair strategies of peripheral nerve injury.

The Hypoxia Response Pathway: A Potential Intervention Target in Parkinson's Disease?

Parkinson's disease (PD) is a progressive neurodegenerative disorder for which only symptomatic treatments are available. Both preclinical and clinical studies suggest that moderate hypoxia induces evolutionarily conserved adaptive mechanisms that enhance neuronal viability and survival. Therefore, targeting the hypoxia response pathway might provide neuroprotection by ameliorating the deleterious effects of mitochondrial dysfunction and oxidative stress, which underlie neurodegeneration in PD. Here, we review experimental studies regarding the link between PD pathophysiology and neurophysiological adaptations to hypoxia. We highlight the mechanistic differences between the rescuing effects of chronic hypoxia in neurodegeneration and short-term moderate hypoxia to improve neuronal resilience, termed "hypoxic conditioning". Moreover, we interpret these preclinical observations regarding the pharmacological targeting of the hypoxia response pathway. Finally, we discuss controversies with respect to the differential effects of hypoxia response pathway activation across the PD spectrum, as well as intervention dosing in hypoxic conditioning and potential harmful effects of such interventions. We recommend that initial clinical studies in PD should focus on the safety, physiological responses, and mechanisms of hypoxic conditioning, as well as on repurposing of existing pharmacological compounds. © 2023 International Parkinson and Movement Disorder Society.

Brief Electrical Stimulation Promotes Recovery after Surgical Repair of Injured Peripheral Nerves

Injured peripheral nerves regenerate their axons in contrast to those in the central nervous system. Yet, functional recovery after surgical repair is often disappointing. The basis for poor recovery is progressive deterioration with time and distance of the growth capacity of the neurons that lose their contact with targets (chronic axotomy) and the growth support of the chronically denervated Schwann cells (SC) in the distal nerve stumps. Nonetheless, chronically denervated atrophic muscle retains the capacity for reinnervation. Declining electrical activity of motoneurons accompanies the progressive fall in axotomized neuronal and denervated SC expression of regeneration-associated-genes and declining regenerative success. Reduced motoneuronal activity is due to the withdrawal of synaptic contacts from the soma. Exogenous neurotrophic factors that promote nerve regeneration can replace the endogenous factors whose expression declines with time. But the profuse axonal outgrowth they provoke and the difficulties in their delivery hinder their efficacy. Brief (1 h) low-frequency (20 Hz) electrical stimulation (ES) proximal to the injury site promotes the expression of endogenous growth factors and, in turn, dramatically accelerates axon outgrowth and target reinnervation. The latter ES effect has been demonstrated in both rats and humans. A conditioning ES of intact nerve days prior to nerve injury increases axonal outgrowth and regeneration rate. Thereby, this form of ES is amenable for nerve transfer surgeries and end-to-side neurorrhaphies. However, additional surgery for applying the required electrodes may be a hurdle. ES is applicable in all surgeries with excellent outcomes.

Hallmarks of peripheral nerve injury and regeneration

Peripheral nerves are functional networks in the body. Disruption of these networks induces varied functional consequences depending on the types of nerves and organs affected. Despite the advances in microsurgical repair and understanding of nerve regeneration biology, restoring full functions after severe traumatic nerve injuries is still far from achieved. While a blunted growth response from axons and errors in axon guidance due to physical barriers may surface as the major hurdles in repairing nerves, critical additional cellular and molecular aspects challenge the orderly healing of injured nerves. Understanding the systematic reprogramming of injured nerves at the cellular and molecular levels, referred to here as "hallmarks of nerve injury regeneration," will offer better ideas. This chapter discusses the hallmarks of nerve injury and regeneration and critical points of failures in the natural healing process. Potential pharmacological and nonpharmacological intervention points for repairing nerves are also discussed.

M. tb Rv0927c suppresses the activation of HIF-1α pathway through VHL-mediated ubiquitination and NF-κB/COX-2 pathway to enhance mycobacteria survival

Mycobacterium tuberculosis (M. tuberculosis), the causative agent of tuberculosis, employs various effector proteins to target and modulate host defenses. Our previous study showed that M. tuberculosis protein Rv0927c can promote the survival of intracellular mycobacteria, but the underlying mechanisms remain poorly understood. Here, we found that Rv0927c inhibited Mycobacterium smegmatis (M. smegmatis) induced hypoxia-inducible factor-1α (HIF-1α) activation in macrophages, and HIF-1α is required for Rv0927c to promote mycobacteria survival. Western blot analysis showed that Rv0927c promoted the proteasomal degradation of HIF-1α via Von Hippel-Lindau (VHL)-mediated ubiquitination and inhibited the nuclear localization of HIF-1α through the NF-κB/COX-2 pathway, thereby suppressing HIF-1α pathway activation. Furthermore, Rv0927c suppressed the host glycolytic metabolism, which is known to be regulated by HIF-1α and depended on the glycolysis process to promote mycobacterial survival. Our findings provide evidence that Rv0927c inhibits the activation of HIF-1α pathway, allowing pathogens to evade host immune responses, suggesting that targeting Rv0927c or HIF-1α might be a potential anti-tuberculosis therapy.

Recent progress and challenges in the treatment of spinal cord injury

Spinal cord injury (SCI) disrupts the structural and functional connectivity between the higher center and the spinal cord, resulting in severe motor, sensory, and autonomic dysfunction with a variety of complications. The pathophysiology of SCI is complicated and multifaceted, and thus individual treatments acting on a specific aspect or process are inadequate to elicit neuronal regeneration and functional recovery after SCI. Combinatory strategies targeting multiple aspects of SCI pathology have achieved greater beneficial effects than individual therapy alone. Although many problems and challenges remain, the encouraging outcomes that have been achieved in preclinical models offer a promising foothold for the development of novel clinical strategies to treat SCI. In this review, we characterize the mechanisms underlying axon regeneration of adult neurons and summarize recent advances in facilitating functional recovery following SCI at both the acute and chronic stages. In addition, we analyze the current status, remaining problems, and realistic challenges towards clinical translation. Finally, we consider the future of SCI treatment and provide insights into how to narrow the translational gap that currently exists between preclinical studies and clinical practice. Going forward, clinical trials should emphasize multidisciplinary conversation and cooperation to identify optimal combinatorial approaches to maximize therapeutic benefit in humans with SCI.

HIF-1α-induced upregulation of m6A reader IGF2BP1 facilitates peripheral nerve injury recovery by enhancing SLC7A11 mRNA stabilization

The recovery of peripheral nerve injury (PNI) is not ideal in clinic. Our previous study revealed that hypoxia treatment promoted PNI repair by inhibiting ferroptosis. The aim of this study was to investigate the underlying molecular mechanism of HIF-1α in hypoxia-PNI recovery. M6A dot blot was used to determine the total level of m6A modification. Besides, HIF-1α small interfering RNA (siRNA) or IGF2BP1 overexpression vector was transfected into dorsal root ganglion (DRG) neurons to alter the expression of HIF-1α and IGF2BP1. Subsequently, MeRIP-PCR analysis was applied to validate the m6A methylation level of SLC7A11. We demonstrated the hypoxia stimulated HIF-1α-dependent expression of IGF2BP1 and promoted the overall m6A methylation levels of DRG neurons. Overexpression of HIF-1α increased the expressions of neurotrophic factors including nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and glial-derived neurotrophic factor (GDNF), which could be effectively reversed by siRNA knockdown of IGF2BP1. Moreover, upregulation of HIF-1α contributed to the m6A methylation level and mRNA stabilization of SLC7A11. This study revealed that the HIF-1α/IGF2BP1/SLC7A11 regulatory axis facilitated the recovery of injured DRG neurons. Our findings suggest a novel insight for the m6A methylation modification in PNI recovery.

Circadian clock regulator Bmal1 gates axon regeneration via Tet3 epigenetics in mouse sensory neurons

Axon regeneration of dorsal root ganglia (DRG) neurons after peripheral axotomy involves reconfiguration of gene regulatory circuits to establish regenerative gene programs. However, the underlying mechanisms remain unclear. Here, through an unbiased survey, we show that the binding motif of Bmal1, a central transcription factor of the circadian clock, is enriched in differentially hydroxymethylated regions (DhMRs) of mouse DRG after peripheral lesion. By applying conditional deletion of Bmal1 in neurons, in vitro and in vivo neurite outgrowth assays, as well as transcriptomic profiling, we demonstrate that Bmal1 inhibits axon regeneration, in part through a functional link with the epigenetic factor Tet3. Mechanistically, we reveal that Bmal1 acts as a gatekeeper of neuroepigenetic responses to axonal injury by limiting Tet3 expression and restricting 5hmC modifications. Bmal1-regulated genes not only concern axon growth, but also stress responses and energy homeostasis. Furthermore, we uncover an epigenetic rhythm of diurnal oscillation of Tet3 and 5hmC levels in DRG neurons, corresponding to time-of-day effect on axon growth potential. Collectively, our studies demonstrate that targeting Bmal1 enhances axon regeneration.

Acute intermittent hypoxia alters disease course and promotes CNS repair including resolution of inflammation and remyelination in the experimental autoimmune encephalomyelitis model of MS

Remyelination and neurodegeneration prevention mitigate disability in Multiple Sclerosis (MS). We have shown acute intermittent hypoxia (AIH) is a novel, non-invasive and effective therapy for peripheral nerve repair, including remyelination. Thus, we posited AIH would improve repair following CNS demyelination and address the paucity of MS repair treatments. AIH's capacity to enhance intrinsic repair, functional recovery and alter disease course in the experimental autoimmune encephalomyelitis (EAE) model of MS was assessed. EAE was induced by MOG35-55 immunization in C57BL/6 female mice. EAE mice received either AIH (10 cycles-5 min 11% oxygen alternating with 5 min 21% oxygen) or Normoxia (control; 21% oxygen for same duration) once daily for 7d beginning at near peak EAE disease score of 2.5. Mice were followed post-treatment for an additional 7d before assessing histopathology or 14d to examine maintenance of AIH effects. Alterations in histopathological correlates of multiple repair indices were analyzed quantitatively in focally demyelinated ventral lumbar spinal cord areas to assess AIH impacts. AIH begun at near peak disease significantly improved daily clinical scores/functional recovery and associated histopathology relative to Normoxia controls and the former were maintained for at least 14d post-treatment. AIH enhanced correlates of myelination, axon protection and oligodendrocyte precursor cell recruitment to demyelinated areas. AIH also effected a dramatic reduction in inflammation, while polarizing remaining macrophages/microglia toward a pro-repair state. Collectively, this supports a role for AIH as a novel non-invasive therapy to enhance CNS repair and alter disease course following demyelination and holds promise as a neuroregenerative MS strategy.

Development of a simultaneous and noninvasive measuring method using high-frame rate videography and motion analysis software for the assessment of facial palsy recovery in a rat model

BACKGROUND

For the development of new therapeutic and reconstructive methods for facial nerve palsy, it is critical to validate them in animal models. This study developed a novel evaluation method using a high-speed camera and motion analysis software for rat facial paralysis models. The validity of the new method was verified using normal rats and rats with facial paralysis.

METHODS

The whisker movement was recorded using a high-frame video camera. The video files were processed using motion analysis software, and the angular velocities were measured. The score was calculated as the percentage of movement on the side that had palsy compared with the movement on the normal side. Normal rats were used to examine which of the four indices of angular velocity is appropriate for this evaluation method. Using this method, two types of facial nerve palsy models were compared. Furthermore, the three agents that were predicted to promote axon regeneration from previous studies were evaluated.

RESULTS

The two averages of the protraction and retraction movement velocities of the whiskers were considered as the most appropriate indicators for this new method. Compared with the saline group, all agent groups showed significant differences in the improvement of facial palsy recovery.

CONCLUSIONS

This method is an evaluation method for the effects of therapeutic intervention for facial nerve paralysis in real time without sacrificing animals.

Extracellular Matrix Regulation in Physiology and in Brain Disease

The extracellular matrix (ECM) surrounds cells in the brain, providing structural and functional support. Emerging studies demonstrate that the ECM plays important roles during development, in the healthy adult brain, and in brain diseases. The aim of this review is to briefly discuss the physiological roles of the ECM and its contribution to the pathogenesis of brain disease, highlighting the gene expression changes, transcriptional factors involved, and a role for microglia in ECM regulation. Much of the research conducted thus far on disease states has focused on "omic" approaches that reveal differences in gene expression related to the ECM. Here, we review recent findings on alterations in the expression of ECM-associated genes in seizure, neuropathic pain, cerebellar ataxia, and age-related neurodegenerative disorders. Next, we discuss evidence implicating the transcription factor hypoxia-inducible factor 1 (HIF-1) in regulating the expression of ECM genes. HIF-1 is induced in response to hypoxia, and also targets genes involved in ECM remodeling, suggesting that hypoxia could contribute to ECM remodeling in disease conditions. We conclude by discussing the role microglia play in the regulation of the perineuronal nets (PNNs), a specialized form of ECM in the central nervous system. We show evidence that microglia can modulate PNNs in healthy and diseased brain states. Altogether, these findings suggest that ECM regulation is altered in brain disease, and highlight the role of HIF-1 and microglia in ECM remodeling.

Self-renewing macrophages in dorsal root ganglia contribute to promote nerve regeneration

Sensory neurons located in dorsal root ganglia (DRG) convey sensory information from peripheral tissue to the brain. After peripheral nerve injury, sensory neurons switch to a regenerative state to enable axon regeneration and functional recovery. This process is not cell autonomous and requires glial and immune cells. Macrophages in the DRG (DRGMacs) accumulate in response to nerve injury, but their origin and function remain unclear. Here, we mapped the fate and response of DRGMacs to nerve injury using macrophage depletion, fate-mapping, and single-cell transcriptomics. We identified three subtypes of DRGMacs after nerve injury in addition to a small population of circulating bone-marrow-derived precursors. Self-renewing macrophages, which proliferate from local resident macrophages, represent the largest population of DRGMacs. The other two subtypes include microglia-like cells and macrophage-like satellite glial cells (SGCs) (Imoonglia). We show that self-renewing DRGMacs contribute to promote axon regeneration. Using single-cell transcriptomics data and CellChat to simulate intercellular communication, we reveal that macrophages express the neuroprotective and glioprotective ligand prosaposin and communicate with SGCs via the prosaposin receptor GPR37L1. These data highlight that DRGMacs have the capacity to self-renew, similarly to microglia in the Central nervous system (CNS) and contribute to promote axon regeneration. These data also reveal the heterogeneity of DRGMacs and their potential neuro- and glioprotective roles, which may inform future therapeutic approaches to treat nerve injury.

Functions of the aryl hydrocarbon receptor (AHR) beyond the canonical AHR/ARNT signaling pathway

The aryl hydrocarbon receptor (AHR) is a ligand-dependent transcription factor regulating adaptive and maladaptive responses toward exogenous and endogenous signals. Research from various biomedical disciplines has provided compelling evidence that the AHR is critically involved in the pathogenesis of a variety of diseases and disorders, including autoimmunity, inflammatory diseases, endocrine disruption, premature aging and cancer. Accordingly, AHR is considered an attractive target for the development of novel preventive and therapeutic measures. However, the ligand-based targeting of AHR is considerably complicated by the fact that the receptor does not always follow the beaten track, i.e. the canonical AHR/ARNT signaling pathway. Instead, AHR might team up with other transcription factors and signaling molecules to shape gene expression patterns and associated physiological or pathophysiological functions in a ligand-, cell- and micromilieu-dependent manner. Herein, we provide an overview about some of the most important non-canonical functions of AHR, including crosstalk with major signaling pathways involved in controlling cell fate and function, immune responses, adaptation to low oxygen levels and oxidative stress, ubiquitination and proteasomal degradation. Further research on these diverse and exciting yet often ambivalent facets of AHR biology is urgently needed in order to exploit the full potential of AHR modulation for disease prevention and treatment.

Deciphering the dynamic niches and regeneration-associated transcriptional program of motoneurons following peripheral nerve injury

Robust axon regeneration of motoneurons (MNs) occurs in rodent models upon peripheral nerve injury (PNI). However, genome-wide dynamic molecules and permissive microenvironment following insult in MNs remain largely unknown. Here, we firstly tackled by high-coverage and massive sequencing of laser-dissected individual ChAT⁺ cells to uncover molecules and pro-regenerative programs of MNs from injury to the regenerating phase after PNI. "Injured" populations at 1d∼7d were well distinguished and three response phases were well defined by elucidating with several clues (Gap43, etc). We found remarkable changes of genes expressed by injured motoneurons to activate and enhance intrinsic axon regrowth or crosstalk with other cellular or non-cellular counterpart in the activated regenerative microenvironment, specifically microglia/macrophage. We also identified an injury and regeneration-associated module and critical regulators including core transcription factors (Atf3, Arid5a, Klf6, Klf7, Jun, Stat3, and Myc). This study provides a vital resource and critical molecules for studying neural repair of axotomized motoneurons.

Analysis of neuronal injury transcriptional response identifies CTCF and YY1 as co-operating factors regulating axon regeneration

Injured sensory neurons activate a transcriptional program necessary for robust axon regeneration and eventual target reinnervation. Understanding the transcriptional regulators that govern this axon regenerative response may guide therapeutic strategies to promote axon regeneration in the injured nervous system. Here, we used cultured dorsal root ganglia neurons to identify pro-regenerative transcription factors. Using RNA sequencing, we first characterized this neuronal culture and determined that embryonic day 13.5 DRG (eDRG) neurons cultured for 7 days are similar to e15.5 DRG neurons in vivo and that all neuronal subtypes are represented. This eDRG neuronal culture does not contain other non-neuronal cell types. Next, we performed RNA sequencing at different time points after in vitro axotomy. Analysis of differentially expressed genes revealed upregulation of known regeneration associated transcription factors, including Jun, Atf3 and Rest, paralleling the axon injury response in vivo. Analysis of transcription factor binding sites in differentially expressed genes revealed other known transcription factors promoting axon regeneration, such as Myc, Hif1α, Pparγ, Ascl1a, Srf, and Ctcf, as well as other transcription factors not yet characterized in axon regeneration. We next tested if overexpression of novel candidate transcription factors alone or in combination promotes axon regeneration in vitro. Our results demonstrate that expression of Ctcf with Yy1 or E2f2 enhances in vitro axon regeneration. Our analysis highlights that transcription factor interaction and chromatin architecture play important roles as a regulator of axon regeneration.

Hypoxia: molecular pathophysiological mechanisms in human diseases

Hypoxia, a low O2 tension, is a fundamental feature that occurs in physiological events as well as pathophysiological conditions, especially mentioned for its role in the mechanism of angiogenesis, glucose metabolism, and cell proliferation/survival. The hypoxic state through the activation of specific mechanisms is an aggravating circumstance commonly noticed in multiple sclerosis, cancer, heart disease, kidney disease, liver disease, lung disease, and in inflammatory bowel disease. On the other hand, hypoxia could play a key role in tissue regeneration and repair of damaged tissues, especially by acting on specific tissue stem cells, but their features may result as a disadvantage when it is concerned for neoplastic stem cells. Furthermore, hypoxia could also have a potential role in tissue engineering and regenerative medicine due to its capacity to improve the performance of biomaterials. The current review aims to highlight the hypoxic molecular mechanisms reported in different pathological conditions to provide an overview of hypoxia as a therapeutic agent in regenerative and molecular therapy.

Graphical abstract

Circuit formation in the adult brain

Neurons in the mammalian central nervous system display an enormous capacity for circuit formation during development but not later in life. In principle, new circuits could be also formed in adult brain, but the absence of the developmental milieu and the presence of growth inhibition and hundreds of working circuits are generally viewed as unsupportive for such a process. Here, we bring together evidence from different areas of neuroscience—such as neurological disorders, adult‐brain neurogenesis, innate behaviours, cell grafting, and in vivo cell reprogramming—which demonstrates robust circuit formation in adult brain. In some cases, adult‐brain rewiring is ongoing and required for certain types of behaviour and memory, while other cases show significant promise for brain repair in disease models. Together, these examples highlight that the adult brain has higher capacity for structural plasticity than previously recognized. Understanding the underlying mechanisms behind this retained plasticity has the potential to advance basic knowledge regarding the molecular organization of synaptic circuits and could herald a new era of neural circuit engineering for therapeutic repair.

The transcription factor C/EBPβ promotes vascular endothelial growth factor A expression and neural stem cell expansion

The bZIP transcription factor CCAAT enhancer-binding protein β (C/EBPβ) exhibits neurogenic, neuritogenic, and pro-survival effects in the central nervous system. Here, we show that C/EBPβ regulates neural stem cell (NSC) expansion and vascular endothelial growth factor A (VEGF-A) level by acting on a C/EBPβ-responsive element within the Vegf-a promoter. As predicted, C/EBPβ depletion reduced VEGF-A production, NSC number, and average neurosphere size in proliferating cultures. Conversely, deletion of the C/EBPβ repressor CHOP-10 induced C/EBPβ and VEGF-A expression, while stimulating NSC expansion. These data highlight the role of C/EBPβ in regulating VEGF-A production and the growth of NSCs and suggest CHOP-dependent antagonism of C/EBPβ may function as a transcriptional rheostat linking stress-associated cues with stem cell quiescence among other pathological responses affecting the neurogenic niche.

Incorporating Blood Flow in Nerve Injury and Regeneration Assessment

Peripheral nerve injury is a significant public health challenge, with limited treatment options and potential lifelong impact on function. More than just an intrinsic part of nerve anatomy, the vascular network of nerves impact regeneration, including perfusion for metabolic demands, appropriate signaling and growth factors, and structural scaffolding for Schwann cell and axonal migration. However, the established nerve injury classification paradigm proposed by Sydney Sunderland in 1951 is based solely on hierarchical disruption to gross anatomical nerve structures and lacks further information regarding the state of cellular, metabolic, or inflammatory processes that are critical in determining regenerative outcomes. This review covers the anatomical structure of nerve-associated vasculature, and describes the biological processes that makes these vessels critical to successful end-organ reinnervation after severe nerve injuries. We then propose a theoretical framework that incorporates measurements of blood vessel perfusion and inflammation to unify perspectives on all mechanisms of nerve injury.

MicroRNA-31 inhibits traumatic brain injury-triggered neuronal cell apoptosis by regulating hypoxia-inducible factor-1A/vascular endothelial growth factor A axis

MicroRNAs are dysregulated in traumatic brain injury and are involved in neuronal cell behaviors. Previous studies identified miR-31 as a spinal cord injury-related microRNA, while its role in traumatic brain injury remains indistinct. Herein, we explored the participation of miR-31 in traumatic brain injury. Traumatic brain injury model was established after traumatic neuron injury. Neurocytes were transfected with miR-31 mimic or inhibitor. Cell counting kit-8, lactate dehydrogenase assay, terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling, and western blot were applied to examine cell viability, lactate dehydrogenase releasing, apoptosis, and apoptosis-related protein. The binding between miR-31 and hypoxia-inducible factor-1A was verified by luciferase assay. Quantitative reverse transcription-PCR was used to detect the regulation of traumatic neuron injury or hypoxia-inducible factor-1A overexpression on vascular endothelial growth factor A level. The effects of hypoxia-inducible factor-1A or vascular endothelial growth factor A on neuronal cell injury were examined. Additionally, phosphatidylinositol 3kinase/AKT/mammalian target of rapamycin (PI3K/AKT/mTOR) pathway was also examined using western blot. Downregulation of miR-31 promoted traumatic neuron injury-induced neuronal cell injury, and its overexpression did the opposite. Hypoxia-inducible factor-1A acted as a downstream mRNA of miR-31 and its downregulation was involved in miR-31-regulated neuronal cell injury. Vascular endothelial growth factor A level was elevated by traumatic neuron injury or hypoxia-inducible factor-1A overexpression. Hypoxia-inducible factor-1A enhanced neuronal cell injury via promoting vascular endothelial growth factor A expression. Furthermore, miR-31/hypoxia-inducible factor-1A/vascular endothelial growth factor A regulated PI3K/AKT/mTOR pathway in neuronal cells. Our study demonstrated miR-31 inhibited neuronal cell apoptosis via regulating hypoxia-inducible factor-1A/vascular endothelial growth factor A axis.

Manganese-Induced Neurotoxicity through Impairment of Cross-Talk Pathways in Human Neuroblastoma Cell Line SH-SY5Y Differentiated with Retinoic Acid

Manganese (Mn) is an important element; yet acute and/or chronic exposure to this metal has been linked to neurotoxicity and neurodegenerative illnesses such as Parkinson’s disease and others via an unknown mechanism. To better understand it, we exposed a human neuroblastoma cell model (SH-SY5Y) to two Mn chemical species, MnCl₂ and Citrate of Mn(II) (0–2000 µM), followed by a cell viability assay, transcriptomics, and bioinformatics. Even though these cells have been chemically and genetically modified, which may limit the significance of our findings, we discovered that by using RA-differentiated cells instead of undifferentiated SH-SY5Y cell line, both chemical species induce a similar toxicity, potentially governed by disruption of protein metabolism, with some differences. The MnCl₂ altered amino acid metabolism, which affects RNA metabolism and protein synthesis. Citrate of Mn(II), however, inhibited the E3 ubiquitin ligases–target protein degradation pathway, which can lead to the buildup of damaged/unfolded proteins, consistent with histone modification. Finally, we discovered that Mn(II)-induced cytotoxicity in RA-SH-SY5Y cells shared 84 percent of the pathways involved in neurodegenerative diseases.

Two-Dimensional Nanomaterials for Peripheral Nerve Engineering: Recent Advances and Potential Mechanisms

Peripheral nerve tissues possess the ability to regenerate within artificial nerve scaffolds, however, despite the advance of biomaterials that support nerve regeneration, the functional nerve recovery remains unsatisfactory. Importantly, the incorporation of two-dimensional nanomaterials has shown to significantly improve the therapeutic effect of conventional nerve scaffolds. In this review, we examine whether two-dimensional nanomaterials facilitate angiogenesis and thereby promote peripheral nerve regeneration. First, we summarize the major events occurring after peripheral nerve injury. Second, we discuss that the application of two-dimensional nanomaterials for peripheral nerve regeneration strategies by facilitating the formation of new vessels. Then, we analyze the mechanism that the newly-formed capillaries directionally and metabolically support neuronal regeneration. Finally, we prospect that the two-dimensional nanomaterials should be a potential solution to long range peripheral nerve defect. To further enhance the therapeutic effects of two-dimensional nanomaterial, strategies which help remedy the energy deficiency after peripheral nerve injury could be a viable solution.

Peptide ligands targeting FGF receptors promote recovery from dorsal root crush injury via AKT/mTOR signaling

Background: Fibroblast growth factor receptors (FGFRs) are key targets for nerve regeneration and repair. The therapeutic effect of exogenous recombinant FGFs in vivo is limited due to their high molecular weight. Small peptides with low molecular weight, easy diffusion, low immunogenicity, and nontoxic metabolite formation are potential candidates. The present study aimed to develop a novel low-molecular-weight peptide agonist of FGFR to promote nerve injury repair.

Methods: Phage display technology was employed to screen peptide ligands targeting FGFR2. The peptide ligand affinity for FGFRs was detected by isothermal titration calorimetry. Structural biology-based computer virtual analysis was used to characterize the interaction between the peptide ligand and FGFR2. The peptide ligand effect on axon growth, regeneration, and behavioral recovery of sensory neurons was determined in the primary culture of sensory neurons and dorsal root ganglia (DRG) explants in vitro and a rat spinal dorsal root injury (DRI) model in vivo. The peptide ligand binding to other membrane receptors was characterized by surface plasmon resonance (SPR) and liquid chromatography-mass spectrometry (LC-MS)/MS. Intracellular signaling pathways primarily affected by the peptide ligand were characterized by phosphoproteomics, and related pathways were verified using specific inhibitors.

Results: We identified a novel FGFR-targeting small peptide, CH02, with seven amino acid residues. CH02 activated FGFR signaling through high-affinity binding with the extracellular segment of FGFRs and also had an affinity for several receptor tyrosine kinase (RTK) family members, including VEGFR2. In sensory neurons cultured in vitro, CH02 maintained the survival of neurons and promoted axon growth. Simultaneously, CH02 robustly enhanced nerve regeneration and sensory-motor behavioral recovery after DRI in rats. CH02-induced activation of FGFR signaling promoted nerve regeneration primarily via AKT and ERK signaling downstream of FGFRs. Activation of mTOR downstream of AKT signaling augmented axon growth potential in response to CH02.

Conclusion: Our study revealed the significant therapeutic effect of CH02 on strengthening nerve regeneration and suggested a strategy for treating peripheral and central nervous system injuries.

Profiling sensory neuron microenvironment after peripheral and central axon injury reveals key pathways for neural repair

Sensory neurons with cell bodies in dorsal root ganglia (DRG) represent a useful model to study axon regeneration. Whereas regeneration and functional recovery occurs after peripheral nerve injury, spinal cord injury or dorsal root injury is not followed by regenerative outcomes. Regeneration of sensory axons in peripheral nerves is not entirely cell autonomous. Whether the DRG microenvironment influences the different regenerative capacities after injury to peripheral or central axons remains largely unknown. To answer this question, we performed a single-cell transcriptional profiling of mouse DRG in response to peripheral (sciatic nerve crush) and central axon injuries (dorsal root crush and spinal cord injury). Each cell type responded differently to the three types of injuries. All injuries increased the proportion of a cell type that shares features of both immune cells and glial cells. A distinct subset of satellite glial cells (SGC) appeared specifically in response to peripheral nerve injury. Activation of the PPARα signaling pathway in SGC, which promotes axon regeneration after peripheral nerve injury, failed to occur after central axon injuries. Treatment with the FDA-approved PPARα agonist fenofibrate increased axon regeneration after dorsal root injury. This study provides a map of the distinct DRG microenvironment responses to peripheral and central injuries at the single-cell level and highlights that manipulating non-neuronal cells could lead to avenues to promote functional recovery after CNS injuries or disease.

Exosomes Secreted by Adipose-Derived Stem Cells Following FK506 Stimulation Reduce Autophagy of Macrophages in Spine after Nerve Crush Injury

Macrophages emerge in the milieu around innervated neurons after nerve injuries. Following nerve injury, autophagy is induced in macrophages and affects the regulation of inflammatory responses. It is closely linked to neuroinflammation, while the immunosuppressive drug tacrolimus (FK506) enhances nerve regeneration following nerve crush injury and nerve allotransplantation with additional neuroprotective and neurotrophic functions. The combined use of FK506 and adipose-derived stem cells (ADSCs) was employed in cell therapy for organ transplantation and vascularized composite allotransplantation. This study aimed to investigate the topical application of exosomes secreted by ADSCs following FK506 treatment (ADSC-F-exo) to the injured nerve in a mouse model of sciatic nerve crush injury. Furthermore, isobaric tags for relative and absolute quantitation (iTRAQ) were used to profile the potential exosomal proteins involved in autophagy. Immunohistochemical analysis revealed that nerve crush injuries significantly induced autophagy in the dorsal root ganglia and dorsal horn of the spinal segments. Locally applied ADSC-F-exo significantly reduced autophagy of macrophages in the spinal segments after nerve crush injury. Proteomic analysis showed that of the 22 abundant exosomal proteins detected in ADSC-F-exo, heat shock protein family A member 8 (HSPA8) and eukaryotic translation elongation factor 1 alpha 1 (EEF1A1) are involved in exosome-mediated autophagy reduction.

Fluorographic findings of diaphragmatic paralysis with spontaneous recovery

BACKGROUND

Postoperative diaphragmatic paralysis is an unavoidable complication of cardiovascular surgery. Although diaphragmatic plication, as a surgical treatment, can be performed, spontaneous recovery is possible. We aimed to identify differences in fluorographic findings of diaphragmatic paralysis between pediatric patients with and without spontaneous recovery within 1 year of intrathoracic surgery.

METHODS

Ten children, who had been followed-up for at least 1 year post-surgery and who had not received diaphragmatic plication were included and classified into those with or without spontaneous recovery. The presence or absence of the paradoxical movement of the diaphragm and mediastinum was evaluated based on fluorographic findings. Fisher's exact test was used to compare the presence or absence of paradoxical movement between the groups.

RESULTS

Eight patients experienced spontaneous recovery. The mean ± standard deviation time to spontaneous recovery was 150 ± 114 days (range, 18-338 days). In the spontaneous recovery group, no patient had paradoxical movement of the mediastinum, and a significant between-group difference was observed in the presence of the paradoxical movement of the mediastinum (present/absent in patients with vs. without spontaneous recovery: 0/8 vs. 2/0, P = 0.02). There was no significant between-group difference in paradoxical movement of the diaphragm (present/absent in patients with vs. without spontaneous recovery: 1/7 vs. 2/0, P = 0.07). Pediatric patients without paradoxical movement of the mediastinum spontaneously recovered within 1 year of intrathoracic surgery.

CONCLUSIONS

Pediatric patients without paradoxical movement of the mediastinum, based on fluorography findings, spontaneously recovered within 1 year of surgery. The timing of spontaneous recovery varied between cases.

Genome-wide chromatin accessibility analyses provide a map for enhancing optic nerve regeneration

Retinal Ganglion Cells (RGCs) lose their ability to grow axons during development. Adult RGCs thus fail to regenerate their axons after injury, leading to vision loss. To uncover mechanisms that promote regeneration of RGC axons, we identified transcription factors (TF) and open chromatin regions that are enriched in rat embryonic RGCs (high axon growth capacity) compared to postnatal RGCs (low axon growth capacity). We found that developmental stage-specific gene expression changes correlated with changes in promoter chromatin accessibility. Binding motifs for TFs such as CREB, CTCF, JUN and YY1 were enriched in the regions of the chromatin that were more accessible in embryonic RGCs. Proteomic analysis of purified rat RGC nuclei confirmed the expression of TFs with potential role in axon growth such as CREB, CTCF, YY1, and JUND. The CREB/ATF binding motif was widespread at the open chromatin region of known pro-regenerative TFs, supporting a role of CREB in regulating axon regeneration. Consistently, overexpression of CREB fused to the VP64 transactivation domain in mouse RGCs promoted axon regeneration after optic nerve injury. Our study provides a map of the chromatin accessibility during RGC development and highlights that TF associated with developmental axon growth can stimulate axon regeneration in mature RGC.

GDNF requires HIF-1α and RET activation for suppression of programmed cell death of enteric neurons by metabolic challenge

Intestinal inflammation challenges both function and structure of the enteric nervous system (ENS). In the animal model of TNBS-induced colitis, an influx of immune cells causes early neuron death in the neuromuscular layers, followed by axonal outgrowth from surviving neurons associated with upregulation of the neurotrophin GDNF (glial cell line-derived neurotrophic factor). Inflammation could involve ischemia and metabolic inhibition leading to neuronal damage, which might be countered by a protective action of GDNF. This was examined in a primary co-culture model of rat myenteric neurons and smooth muscle, where metabolic challenge was caused by dinitrophenol (DNP), O-methyl glucose (OMG) or hypoxia. These caused the specific loss of 50% of neurons by 24 h that was blocked by GDNF both in vitro and in whole mounts. Neuroprotection was lost with RET inhibition by vandetanib or GSK3179106, which also caused neuron loss in untreated controls. Thus, both basal and upregulated GDNF levels signal via RET for neuronal survival. This includes a key role for upregulation of HIF-1α, which was detected in neurons in colitis, since the inhibitor chetomin blocked rescue by GDNF or ischemic pre-conditioning in vitro. In DNP-treated co-cultures, neuron death was not inhibited by zVAD, necrosulfonamide or GSK872, and cleaved caspase-3 or - 8 were undetectable. However, combinations of inhibitors or the RIP1kinase inhibitor Nec-1 prevented neuronal death, evidence for RIPK1-dependent necroptosis. Therefore, inflammation challenges enteric neurons via ischemia, while GDNF is neuroprotective, activating RET and HIF-1α to limit programmed cell death. This may support novel strategies to address recurrent inflammation in IBD.

Biomimicry in 3D printing design: implications for peripheral nerve regeneration

Nerve guide conduits (NGCs) connect dissected nerve stumps and effectively repair short-range peripheral nerve defects. However, for long-range defects, autografts show better therapeutic effects, despite intrinsic limitations. Recent evidence shows that biomimetic design is essential for high-performance NGCs, and 3D printing is a promising fabricating technique. The current work includes a brief review of the challenges for peripheral nerve regeneration. The authors propose a potential solution using biomimetic 3D-printed NGCs as alternative therapies. The assessment of biomimetic designs includes microarchitecture, mechanical property, electrical conductivity and biologics inclusion. The applications of 3D printing in preparing NGCs and present strategies to improve therapeutic effects are also discussed.

COMSOL Multiphysics® modelling of oxygen diffusion through a cellulose nanofibril conduit employed for peripheral nerve repair

BACKGROUND

Peripheral nerve injury can cause significant impairment, and the current methods for facilitating repair, particularly over distances greater than approximately 1 mm, are not entirely effective. Allografts, autografts, and synthetic conduits are three of the most common surgical interventions for peripheral nerve repair; however, each has limitations including poor biocompatibility, adverse immune responses, and the need for successive surgeries. A potential new method for promoting peripheral nerve repair that addresses the shortcomings of current interventions is a biocompatible cellulose nanofibril (CNF) conduit that degrades in-vivo over time. Preliminary testing in multiple animal models has yielded positive results, but more information is needed regarding how the CNF conduit facilitates nutrient and gas flow.

RESULTS

The current work employs 3D modelling and analysis via COMSOL Multiphysics® to determine how the CNF conduit facilitates oxygen movement both radially through the conduit walls and axially along the length of the conduit. Various CNF wall permeabilities, conduit lengths, and nerve-to-conduit diameter ratios have been examined; all of which were shown to have an impact on the resultant oxygen profile within the conduit. When the walls of the CNF conduit were modeled to have significant oxygen permeability, oxygen diffusion across the conduit was shown to dominate relative to axial diffusion of oxygen along the length of the conduit, which was otherwise the controlling diffusion mechanism.

CONCLUSIONS

The results of this study suggest that there is a complex relationship between axial and radial diffusion as the properties of the conduit such as length, diameter, and permeability are altered and when investigating various locations within the model. At low wall permeabilities the axial diffusion is dominant for all configurations, while for higher wall permeabilities the radial diffusion became dominant for smaller diameters. The length of the conduit did not alter the mechanism of diffusion, but rather had an inverse relationship with the magnitude of the overall concentration profile. As such the modeling results may be employed to predict and control the amount and distribution of oxygenation throughout the conduit, and hence to guide experimental conduit design.

LncRNA MIAT activates vascular endothelial growth factor A through RAD21 to promote nerve injury repair in acute spinal cord injury

BACKGROUND

The main pathological feature of acute spinal cord injury (ASCI) is neuronal apoptosis and Long non-coding RNA (lncRNA) myocardial infarction-related transcript (MIAT) is involved in the regulation of neuronal apoptosis. This study aimed to investigate the role and potential mechanism of LncRNA MIAT in neuronal apoptosis induced by ASCI.

METHODS

After Lenti-MIAT lentivirus was microinjected into ASCI rats, Basso, Beattie and Bresnahan Score, Hematoxylin-eosin staining, TUNEL staining, immunohistochemical, immunofluorescence, quantitative real-time PCR and Western blot were used to observe the effect of LncRNA MIAT on the nerve function of ASCI rats. MTT and flow cytometry assays were used to identify the in vitro function of LncRNA MIAT. RNA immunoprecipitation, RNA pull-down, Cycloheximide chase and Chromatin immunoprecipitation combined with qPCR experiments were used to study the mechanism.

RESULTS

The overexpression of LncRNA MIAT was conducive to the recovery of motor function in ASCI rats and repressed neuronal cell apoptosis and increased neuronal cell viability. Furthermore, the overexpression of LncRNA MIAT in PC12 cells upregulated RAD21 expression by repressing RAD21 protein degradation and further promoted VEGFA transcription to inhibit neuronal cell apoptosis, ultimately improved ASCI.

CONCLUSION

Our data indicated that the overexpression of LncRNA MIAT activated VEGFA through RAD21 to inhibit neuronal cell apoptosis in ASCI.

Acute intermittent hypoxia enhances regeneration of surgically repaired peripheral nerves in a manner akin to electrical stimulation

The intrinsic repair response of injured peripheral neurons is enhanced by brief electrical stimulation (ES) at time of surgical repair, resulting in improved regeneration in rodents and humans. However, ES is invasive. Acute intermittent hypoxia (AIH) - breathing alternate cycles of regular air and air with ~50% normal oxygen levels (11% O₂), considered mild hypoxia, is an emerging, promising non-invasive therapy that promotes motor function in spinal cord injured rats and humans. AIH can increase neural activity and under moderately severe hypoxic conditions improves repair of peripherally crushed nerves in mice. Thus, we posited an AIH paradigm similar to that used clinically for spinal cord injury, will improve surgically repaired peripheral nerves akin to ES, including an impact on regeneration-associated gene (RAG) expression-a predictor of growth states. Alterations in early RAG expression were examined in adult male Lewis rats that underwent tibial nerve coaptation repair with either 2 days AIH or normoxia control treatment begun on day 2 post-repair, or 1 h ES treatment (20 Hz) at time of repair. Three days post-repair, AIH or ES treatments effected significant and parallel elevated RAG expression relative to normoxia control at the level of injured sensory and motor neuron cell bodies and proximal axon front. These parallel impacts on RAG expression were coupled with significant improvements in later indices of regeneration, namely enhanced myelination and increased numbers of newly myelinated fibers detected 20 mm distal to the tibial nerve repair site or sensory and motor neurons retrogradely labeled 28 mm distal to the repair site, both at 25 days post nerve repair; and improved return of toe spread function 5-10 weeks post-repair. Collectively, AIH mirrors many beneficial effects of ES on peripheral nerve repair outcomes. This highlights its potential for clinical translation as a non-invasive means to effect improved regeneration of injured peripheral nerves.

PDK1 is a negative regulator of axon regeneration

Axon regeneration in the central nervous system is inefficient. However, the neurons in the peripheral nervous system display robust regeneration after injury, indicating that axonal regeneration is differentially controlled under various conditions. To identify those molecules regulating axon regeneration, comparative analysis from dorsal root ganglion neurons at embryonic or adult stages is utilized, which reveals that PDK1 is functions as a negative regulator of axon regeneration. PDK1 is downregulated in embryonic neurons after axotomy. In contrast, sciatic nerve axotomy upregulated PDK1 at protein levels from adult mice. The knockdown of PDK1 or the chemical inhibition of PDK1 promotes axon regeneration in vitro and in vivo. Here we present PDK1 as a new player to negatively regulate axon regeneration and as a potential target in the development of therapeutic applications.

Factors regulating axon regeneration via JNK MAP kinase in Caenorhabditis elegans

Axon regeneration following nerve injury is a highly conserved process in animals. The nematode Caenorhabditis elegans is an excellent model for investigating the molecular mechanisms of axon regeneration. Recent studies using C. elegans have shown that the c-Jun N-terminal kinase (JNK) plays the important role in axon regeneration. Furthermore, many factors have been identified that act upstream of the JNK cascade after axotomy. This review introduces these factors and describes their roles during the regulation of axon regeneration.

Unlocking mammalian regeneration through hypoxia inducible factor one alpha signaling

Historically, the field of regenerative medicine has aimed to heal damaged tissue through the use of biomaterials scaffolds or delivery of foreign progenitor cells. Despite 30 years of research, however, translation and commercialization of these techniques has been limited. To enable mammalian regeneration, a more practical approach may instead be to develop therapies that evoke endogenous processes reminiscent of those seen in innate regenerators. Recently, investigations into tadpole tail regrowth, zebrafish limb restoration, and the super-healing Murphy Roths Large (MRL) mouse strain, have identified ancient oxygen-sensing pathways as a possible target to achieve this goal. Specifically, upregulation of the transcription factor, hypoxia-inducible factor one alpha (HIF-1α) has been shown to modulate cell metabolism and plasticity, as well as inflammation and tissue remodeling, possibly priming injuries for regeneration. Since HIF-1α signaling is conserved across species, environmental or pharmacological manipulation of oxygen-dependent pathways may elicit a regenerative response in non-healing mammals. In this review, we will explore the emerging role of HIF-1α in mammalian healing and regeneration, as well as attempts to modulate protein stability through hyperbaric oxygen treatment, intermittent hypoxia therapy, and pharmacological targeting. We believe that these therapies could breathe new life into the field of regenerative medicine.

In Vivo Gene Delivery of STC2 Promotes Axon Regeneration in Sciatic Nerves

Neurons are vulnerable to injury, and failure to activate self-protective systems after injury leads to neuronal death. However, sensory neurons in dorsal root ganglions (DRGs) mostly survive and regenerate their axons. To understand the mechanisms of the neuronal injury response, we analyzed the injury-responsive transcriptome and found that Stc2 is immediately upregulated after axotomy. Stc2 is required for axon regeneration in vivo and in vitro, indicating that Stc2 is a neuronal factor regulating axonal injury response. The application of the secreted stanniocalcin 2 to injured DRG neurons promotes regeneration. Stc2 thus represents a potential secretory protein with a feedback function regulating regeneration. Finally, the in vivo gene delivery of STC2 increases regenerative growth after injury in peripheral nerves in mice. These results suggest that Stc2 is an injury-responsive gene required for axon regeneration and a potential target for developing therapeutic applications.

Ascending dorsal column sensory neurons respond to spinal cord injury and downregulate genes related to lipid metabolism

Regeneration failure after spinal cord injury (SCI) results in part from the lack of a pro-regenerative response in injured neurons, but the response to SCI has not been examined specifically in injured sensory neurons. Using RNA sequencing of dorsal root ganglion, we determined that thoracic SCI elicits a transcriptional response distinct from sciatic nerve injury (SNI). Both SNI and SCI induced upregulation of ATF3 and Jun, yet this response failed to promote growth in sensory neurons after SCI. RNA sequencing of purified sensory neurons one and three days after injury revealed that unlike SNI, the SCI response is not sustained. Both SCI and SNI elicited the expression of ATF3 target genes, with very little overlap between conditions. Pathway analysis of differentially expressed ATF3 target genes revealed that fatty acid biosynthesis and terpenoid backbone synthesis were downregulated after SCI but not SNI. Pharmacologic inhibition of fatty acid synthase, the enzyme generating palmitic acid, decreased axon growth and regeneration in vitro. These results support the notion that decreased expression of lipid metabolism-related genes after SCI, including fatty acid synthase, may restrict axon regenerative capacity after SCI.

The Mechanisms of Peripheral Nerve Preconditioning Injury on Promoting Axonal Regeneration

Two major factors contribute to the failure of axonal regrowth in the central nervous system (CNS), namely, the neuronal intrinsic regenerative capacity and the extrinsic local inhibitory microenvironments. However, a preconditioning peripheral nerve lesion could substantially enhance the regeneration of central axons following a subsequent spinal cord injury. In the present review, we summarize the molecular mechanisms of the preconditioning injury effect on promoting axonal regeneration. The injury signal transduction resulting from preconditioning peripheral nerve injury regulates the RAG expression to enhance axonal regeneration. Importantly, preconditioning peripheral nerve injury triggers interactions between neurons and nonneuronal cells to amplify and maintain their effects. Additionally, the preconditioning injury impacts mitochondria, protein, and lipid synthesis. All these coordinated changes endow axonal regeneration.

Transcriptome analysis of molecular mechanisms underlying facial nerve injury repair in rats

Although the transcriptional alterations inside the facial nucleus after facial nerve injury have been well studied, the gene expression changes in the facial nerve trunk after injury are still unknown. In this study, we established an adult rat model of facial nerve crush injury by compressing the right lateral extracranial nerve trunk. Transcriptome sequencing, differential gene expression analysis, and cluster analysis of the injured facial nerve trunk were performed, and 39 intersecting genes with significant variance in expression were identified. Gene Ontology annotation and Kyoto Encyclopedia of Genes and Genomes pathway analyses of the 39 intersecting genes revealed that these genes are mostly involved in leukocyte cell-cell adhesion and phagocytosis and have essential roles in regulating nerve repair. Quantitative real-time polymerase chain reaction assays were used to validate the expression of pivotal genes. Finally, nine pivotal genes that contribute to facial nerve recovery were identified, including Arhgap30, Akr1b8, C5ar1, Csf2ra, Dock2, Hcls1, Inpp5d, Sla, and Spi1. Primary Schwann cells were isolated from the sciatic nerve of neonatal rats. After knocking down Akr1b8 in Schwann cells with an Akr1b8-specific small interfering RNA plasmid, expression levels of monocyte chemoattractant protein-1 and interleukin-6 were decreased, while cell proliferation and migration were not obviously altered. These findings suggest that Akr1b8 likely regulates the interaction between Schwann cells and macrophages through regulation of cytokine expression to promote facial nerve regeneration. This study is the first to reveal a transcriptome change in the facial nerve trunk after facial nerve injury, thereby revealing the potential mechanism underlying repair of facial nerve injury. This study was approved by the Animal Ethics Committee of Nantong University, China in 2018 (approval No. S20180923-007).